Around May 17-20 the Mud Rift feature at Waimangu Geothermal area erupted. The Mud Rift is a 36 m long, 5-6 m wide and 15 m deep geothermal vent in Raupo Pond Crater, one of the craters formed in June 1886. It is an isolated area and this is the first known eruption since 1981. The Mud Rift formed in 1906 and was last active in 1978/79 and 1981. However when visited last week the hottest temperature on the floor of the rift was only 13.7 ?C in a pool of brown water.

The May 2016 eruption was from 3 sources, the most active been the two smaller ‘vents/collapse pits’ to the west of the main Mud Rift. These were formed in 1981. We have not been able to see anything on our nearest seismographs. It appears to be mainly steam and fine sands-muds involved. There is lots of evidence of fluids flooding in to rift, especially at the west end. Vegetation nearby is stressed and browning off, but is not damaged by the eruption (no broken limbs etc). There is also some evidence of collapse around the edges of the two smaller vents. Over all it appears as a very short lived event, steam driven with a lot of water also involved (think muddy geyser).

There are two large crater lakes nearby; Inferno Crater Lake and Frying Pan Lake. These are jointly monitoring by GeoNet and the BOP and Waikato Regional Council’s. Inferno Crater is remarkable in that the water level fluctuates through 6-9 meters and over flows about every 40 days. Meanwhile Frying Pan Lake is always flowing, today around 80 litres per sec. In the 1980’s it was closer to 100 litres per sec. These crater lakes display a unique interaction, as the water level of Inferno falls the flow from Frying Pan increases and vice versa. This is termed ‘exchange of function’ and is often observed in hot springs.

From January through to early April 2016 Inferno Crater Lake experienced a ‘high stand’, many of these have occurred since the 1970’s when detailed monitoring started of the lakes. During these times more steam energy is present in the Inferno portion of the geothermal system. This was particularily noticed in 1978/79 and 1981. It would appear this increase of steam through the system was responsible for driving the reactivation of the Mud Rift in May 2016. We have located 47 earthquakes within about 10 km of Waimangu in the last year. In the last 6 years we have located 388, which is typically 64 per year.